1. Field of Invention
This invention relates to calibration of visual measuring systems using a two-point cold calibration technique. More specifically, this invention relates to calibration of CCD cameras in a crystal growing apparatus and a calibration device for such an apparatus.
2. Description of Related Art
A substantial majority of monocrystalline silicon used to make silicon wafers for the microelectronics industry is produced by crystal growing apparatus using the well-known Czochralski process. The Czochralski process basically involves melting high-purity polycrystalline silicon in a quartz crucible in a specially designed furnace to form a silicon melt. A small seed crystal is suspended above the silicon melt on a pull wire or the like, which is arranged to be raised and lowered in a generally vertical direction. The seed crystal is lowered into contact with the silicon melt. The seed crystal is then raised slowly from the silicon melt so that a silicon crystal rod is grown by drawing silicon from the silicon melt. Examples of Czochralski crystal growing systems are described in U.S. Pat. Nos. 5,406,905; 5,911,825; and 5,976,245, each of which is incorporated herein by reference in its entirety.
Various control parameters, such as melt temperature, melt level and rate of crystal pulling, are known to affect the shape and size of the crystal rod being grown. Accordingly, the various parameters must be monitored and controlled to achieve a desired monosilicon crystal. Variations in the diameter of the crystal rod may result in excessive waste because the crystal rod must be trimmed and cut to produce silicon crystal wafer slices of uniform physical dimensions. Typically, the crystal rod is subjected to a grinding process to remove the outer peripheral surface until a predetermined diameter is obtained. Because of purity concerns, the material ground from the crystal rod cannot be reused. Accordingly, any material ground from the crystal rod is waste, resulting in undesirable increases in production cost when the diameter of the crystal rod is not accurately controlled during crystal growth.
It is known to monitor the diameter of a crystal during growth to achieve a desired crystal size. The various control parameters are monitored and adjusted to maintain the desired diameter during crystal growth. It is known to accurately measure the diameter of a growing crystal using CCD camera technology. For example, U.S. Pat. Nos. 5,437,242; 5,653,799; 5,660,629; 5,656,078; 5,665,159; 5,846,318; 5,961,716; and 6,030,451, each of which is incorporated herein by reference in its entirety, disclose various methods and apparatus for controlling the Czochralski process using one or more CCD cameras. Visual measuring systems employing other types of width analyzing apparatus are known and used, such as optical pyrometers, photocells, rotating mirrors with photocells, light sources with photocells, line-scan cameras, and two-dimensional array cameras.
Such visual measuring systems must be calibrated. Typically, calibration is performed by an operator and must rely on the skill of the operator. A known cold calibration technique for crystal growing apparatus involves growing several crystals of various diameters and recording data. Then, using the data and a two-point calibration formula, a two-point calibration is performed for the system. Such two-point calibration is used whenever a crystal growing apparatus is used to grow crystal rods with different diameters.
The invention is based upon the realization that the current two-point cold calibration technique is unsatisfactory because it does not take into account non-linear error. Further, it would be desirable to be able to calibrate the visual measuring system of a crystal growing apparatus without the use of expensive, high-purity silicon. In other words, it would be desirable to achieve calibration without actually growing crystal rods during the calibration process.
One aspect of this invention provides a calibration method for a visual measuring system that takes into account non-linear error. Another aspect of this invention achieves calibration of a visual measuring system of a crystal growing apparatus without the use of expensive, high-purity silicon in the calibration process. This invention provides improved diameter control for a crystal growing apparatus using a visual measuring system.
The calibration method according one aspect of this invention involves calculating an offset for the visual measuring system. The offset O is calculated using the following formula:
O=Zxc2x7k/G,
where Z is a zero error value, k is an internal constant of the width analyzer of the visual measuring system, and G is the gain of the visual measuring system. The zero error value Z is calculated from data using the following formula:
Z=Cxe2x88x92A/((Axe2x88x92a)/(Cxe2x88x92c))
where C is a first linear distance measured by the visual measuring system, A is the actual linear distance corresponding to C, c is a second linear distance measured by the visual measuring system, and a is the actual linear distance corresponding to c.
In one exemplary embodiment of the calibration method according to this aspect of the invention, a reference point is selected relative to a measurement standard. The gain of the measurement system is set so that an output signal is equal to a predetermined value while measuring a first point at a first linear distance from the reference point. The first linear distance of the first point is measured by the visual measurement system. A second point at a second linear distance from the reference point, less than the first linear distance, is measured by the visual measurement system. The actual values of the first and second linear distances are determined by reading the measurement standard. In the zero error formula above, the measured first linear distance of the first point is C, the actual first linear distance of the first point is A, the measured second linear distance of the second point is c and the actual second linear distance of the second point is a. The zero error Z is thus calculated and used in the offset formula above along with the value of the gain G and the internal constant k of the width analyzer of the visual measuring system to obtain the offset O.
Another aspect of this invention provides a calibration device which is used to carry out a calibration method in an apparatus using a visual measuring system including a width analyzer. The calibration device according to this aspect of the invention comprises a measurement standard that is mountable in a self-aligning manner within the apparatus. In particular, the measurement standard may be attached to a vertical support member that is mountable in a self-aligning manner within the apparatus.
According to various exemplary embodiments, the measurement standard may be a ruler and may be arranged to extend in a direction orthogonal to the vertical support member. Further, the measurement standard may be attached to the vertical support member by a countersunk screw which is centered at a zero position on the measurement standard.
According to various exemplary embodiments, the vertical support member may comprise a pedestal plug having a relatively wide proximal end and a relatively narrow distal end. The pedestal plug preferably is made of graphite and may have a tapered section disposed between the proximal end and the distal end.
The vertical support member may further comprise a vertical spacer extending between the pedestal plug and the measurement standard such that the measurement standard is suspended at a predetermined distance above the pedestal plug. Preferably, the vertical spacer is narrower than the distal end of the pedestal plug.
These and other features and advantages of this invention are described in or are apparent from the following detailed description of various exemplary embodiments of the calibration method and device according to this invention.